package com.lpf.jdk6;

import java.util.*;

public class LinkedList<E>
        extends AbstractSequentialList<E>
        implements List<E>, Deque<E>, Cloneable, java.io.Serializable {

    // 链表的表头，表头不包含任何数据。Entry是个链表类数据结构。
    private transient Entry<E> header = new Entry<E>(null, null, null);
    // LinkedList中元素个数
    private transient int size = 0;

    // 默认构造函数：创建一个空的链表
    public LinkedList() {
        header.next = header.previous = header;
    }

    // 包含“集合”的构造函数:创建一个包含“集合”的LinkedList
    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

    // 获取LinkedList的第一个元素
    public E getFirst() {
        if (size == 0)
            throw new NoSuchElementException();
        // 链表的表头header中不包含数据。
        // 这里返回header所指下一个节点所包含的数据。
        return header.next.element;
    }

    // 获取LinkedList的最后一个元素
    public E getLast() {
        if (size == 0)
            throw new NoSuchElementException();
        // 由于LinkedList是双向链表；而表头header不包含数据。
        // 因而，这里返回表头header的前一个节点所包含的数据。
        return header.previous.element;
    }

    // 删除LinkedList的第一个元素
    public E removeFirst() {
        return remove(header.next);
    }

    // 删除LinkedList的最后一个元素
    public E removeLast() {
        return remove(header.previous);
    }

    // 将元素添加到LinkedList的起始位置
    public void addFirst(E e) {
        addBefore(e, header.next);
    }

    // 将元素添加到LinkedList的结束位置
    public void addLast(E e) {
        addBefore(e, header);
    }

    // 判断LinkedList是否包含元素(o)
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    // 返回LinkedList的大小
    public int size() {
        return size;
    }

    // 将元素(E)添加到LinkedList中
    public boolean add(E e) {
        // 将节点(节点数据是e)添加到表头(header)之前。
        // 即，将节点添加到双向链表的末端。
        addBefore(e, header);
        return true;
    }

    // 从LinkedList中删除元素(o)
    // 从链表开始查找，如存在元素(o)则删除该元素并返回true；
    // 否则，返回false。
    public boolean remove(Object o) {
        if (o == null) {
            // 若o为null的删除情况
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (e.element == null) {
                    remove(e);
                    return true;
                }
            }
        } else {
            // 若o不为null的删除情况
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (o.equals(e.element)) {
                    remove(e);
                    return true;
                }
            }
        }
        return false;
    }

    // 将“集合(c)”添加到LinkedList中。
    // 实际上，是从双向链表的末尾开始，将“集合(c)”添加到双向链表中。
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    // 从双向链表的index开始，将“集合(c)”添加到双向链表中。
    public boolean addAll(int index, Collection<? extends E> c) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: " + index +
                    ", Size: " + size);
        Object[] a = c.toArray();
        // 获取集合的长度
        int numNew = a.length;
        if (numNew == 0)
            return false;
        modCount++;

        // 设置“当前要插入节点的后一个节点”
        Entry<E> successor = (index == size ? header : entry(index));
        // 设置“当前要插入节点的前一个节点”
        Entry<E> predecessor = successor.previous;
        // 将集合(c)全部插入双向链表中
        for (int i = 0; i < numNew; i++) {
            Entry<E> e = new Entry<E>((E) a[i], successor, predecessor);
            predecessor.next = e;
            // 下一个要插入的元素插在 e 的后面
            predecessor = e;
        }
        successor.previous = predecessor;

        // 调整LinkedList的实际大小
        size += numNew;
        return true;
    }

    // 清空双向链表
    public void clear() {
        Entry<E> e = header.next;
        // 从表头开始，逐个向后遍历；对遍历到的节点执行以下操作：
        // (01) 设置前一个节点为null
        // (02) 设置当前节点的内容为null
        // (03) 设置后一个节点为“新的当前节点”
        while (e != header) {
            Entry<E> next = e.next;
            e.next = e.previous = null;
            e.element = null;
            e = next;
        }
        header.next = header.previous = header;
        // 设置大小为0
        size = 0;
        modCount++;
    }


    // Positional Access Operations

    // 返回LinkedList指定位置的元素
    public E get(int index) {
        return entry(index).element;
    }

    // 设置index位置对应的节点的值为element
    public E set(int index, E element) {
        Entry<E> e = entry(index);
        E oldVal = e.element;
        e.element = element;
        return oldVal;
    }

    // 在index前添加节点，且节点的值为element
    public void add(int index, E element) {
        addBefore(element, (index == size ? header : entry(index)));
    }

    // 删除index位置的节点
    public E remove(int index) {
        return remove(entry(index));
    }

    // 获取双向链表中指定位置的节点
    private Entry<E> entry(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("Index: " + index +
                    ", Size: " + size);
        Entry<E> e = header;
        // 获取index处的节点。
        // 若index < 双向链表长度的1/2,则从前向后查找;
        // 否则，从后向前查找。
        if (index < (size >> 1)) {
            for (int i = 0; i <= index; i++)
                e = e.next;
        } else {
            for (int i = size; i > index; i--)
                e = e.previous;
        }
        return e;
    }


    // Search Operations

    // 从前向后查找，返回“值为对象(o)的节点对应的索引”
    // 不存在就返回-1
    public int indexOf(Object o) {
        int index = 0;
        if (o == null) {
            for (Entry e = header.next; e != header; e = e.next) {
                if (e.element == null)
                    return index;
                index++;
            }
        } else {
            for (Entry e = header.next; e != header; e = e.next) {
                if (o.equals(e.element))
                    return index;
                index++;
            }
        }
        return -1;
    }

    // 从后向前查找，返回“值为对象(o)的节点对应的索引”
    // 不存在就返回-1
    public int lastIndexOf(Object o) {
        int index = size;
        if (o == null) {
            for (Entry e = header.previous; e != header; e = e.previous) {
                index--;
                if (e.element == null)
                    return index;
            }
        } else {
            for (Entry e = header.previous; e != header; e = e.previous) {
                index--;
                if (o.equals(e.element))
                    return index;
            }
        }
        return -1;
    }

    // Queue operations.

    // 返回第一个节点
    // 若LinkedList的大小为0,则返回null
    public E peek() {
        if (size == 0)
            return null;
        return getFirst();
    }

    // 返回第一个节点
    // 若LinkedList的大小为0,则抛出异常
    public E element() {
        return getFirst();
    }

    // 删除并返回第一个节点
    // 若LinkedList的大小为0,则返回null
    public E poll() {
        if (size == 0)
            return null;
        return removeFirst();
    }

    /**
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E remove() {
        return removeFirst();
    }

    // 将e添加双向链表末尾
    public boolean offer(E e) {
        return add(e);
    }

    // Deque operations

    // 将e添加双向链表开头
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    // 将e添加双向链表末尾
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

    // 返回第一个节点
    // 若LinkedList的大小为0,则返回null
    public E peekFirst() {
        if (size == 0)
            return null;
        return getFirst();
    }

    // 返回最后一个节点
    // 若LinkedList的大小为0,则返回null
    public E peekLast() {
        if (size == 0)
            return null;
        return getLast();
    }

    // 删除并返回第一个节点
    // 若LinkedList的大小为0,则返回null
    public E pollFirst() {
        if (size == 0)
            return null;
        return removeFirst();
    }

    // 删除并返回最后一个节点
    // 若LinkedList的大小为0,则返回null
    public E pollLast() {
        if (size == 0)
            return null;
        return removeLast();
    }

    // 将e插入到双向链表开头
    public void push(E e) {
        addFirst(e);
    }

    // 删除并返回第一个节点
    public E pop() {
        return removeFirst();
    }

    // 从LinkedList开始向后查找，删除第一个值为元素(o)的节点
    // 从链表开始查找，如存在节点的值为元素(o)的节点，则删除该节点
    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }

    // 从LinkedList末尾向前查找，删除第一个值为元素(o)的节点
    // 从链表开始查找，如存在节点的值为元素(o)的节点，则删除该节点
    public boolean removeLastOccurrence(Object o) {
        if (o == null) {
            for (Entry<E> e = header.previous; e != header; e = e.previous) {
                if (e.element == null) {
                    remove(e);
                    return true;
                }
            }
        } else {
            for (Entry<E> e = header.previous; e != header; e = e.previous) {
                if (o.equals(e.element)) {
                    remove(e);
                    return true;
                }
            }
        }
        return false;
    }

    // 返回“index到末尾的全部节点”对应的ListIterator对象(List迭代器)
    public ListIterator<E> listIterator(int index) {
        return new ListItr(index);
    }

    // List迭代器
    private class ListItr implements ListIterator<E> {
        // 上一次返回的节点
        private Entry<E> lastReturned = header;
        // 下一个节点
        private Entry<E> next;
        // 下一个节点对应的索引值
        private int nextIndex;
        // 期望的改变计数。用来实现fail-fast机制。
        private int expectedModCount = modCount;

        // 构造函数。
        // 从index位置开始进行迭代
        ListItr(int index) {
            // index的有效性处理
            if (index < 0 || index > size)
                throw new IndexOutOfBoundsException("Index: " + index +
                        ", Size: " + size);
            // 若 “index 小于 ‘双向链表长度的一半’”，则从第一个元素开始往后查找；
            // 否则，从最后一个元素往前查找。
            if (index < (size >> 1)) {
                next = header.next;
                for (nextIndex = 0; nextIndex < index; nextIndex++)
                    next = next.next;
            } else {
                next = header;
                for (nextIndex = size; nextIndex > index; nextIndex--)
                    next = next.previous;
            }
        }

        // 是否存在下一个元素
        public boolean hasNext() {
            // 通过元素索引是否等于“双向链表大小”来判断是否达到最后。
            return nextIndex != size;
        }

        // 获取下一个元素
        public E next() {
            checkForComodification();
            if (nextIndex == size)
                throw new NoSuchElementException();

            lastReturned = next;
            // next指向链表的下一个元素
            next = next.next;
            nextIndex++;
            return lastReturned.element;
        }

        // 是否存在上一个元素
        public boolean hasPrevious() {
            // 通过元素索引是否等于0，来判断是否达到开头。
            return nextIndex != 0;
        }

        // 获取上一个元素
        public E previous() {
            if (nextIndex == 0)
                throw new NoSuchElementException();

            lastReturned = next = next.previous;
            nextIndex--;
            checkForComodification();
            return lastReturned.element;
        }

        public int nextIndex() {
            return nextIndex;
        }

        public int previousIndex() {
            return nextIndex - 1;
        }

        public void remove() {
            checkForComodification();
            Entry<E> lastNext = lastReturned.next;
            try {
                LinkedList.this.remove(lastReturned);
            } catch (NoSuchElementException e) {
                throw new IllegalStateException();
            }
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = header;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == header)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.element = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = header;
            addBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    private static class Entry<E> {
        E element;
        Entry<E> next;
        Entry<E> previous;

        Entry(E element, Entry<E> next, Entry<E> previous) {
            this.element = element;
            this.next = next;
            this.previous = previous;
        }
    }

    private Entry<E> addBefore(E e, Entry<E> entry) {
        Entry<E> newEntry = new Entry<E>(e, entry, entry.previous);
        newEntry.previous.next = newEntry;
        newEntry.next.previous = newEntry;
        size++;
        modCount++;
        return newEntry;
    }

    private E remove(Entry<E> e) {
        if (e == header)
            throw new NoSuchElementException();

        E result = e.element;
        e.previous.next = e.next;
        e.next.previous = e.previous;
        e.next = e.previous = null;
        e.element = null;
        size--;
        modCount++;
        return result;
    }

    /**
     * @since 1.6
     */
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    /**
     * Adapter to provide descending iterators via ListItr.previous
     */
    private class DescendingIterator implements Iterator {
        final ListItr itr = new ListItr(size());

        public boolean hasNext() {
            return itr.hasPrevious();
        }

        public E next() {
            return itr.previous();
        }

        public void remove() {
            itr.remove();
        }
    }

    /**
     * Returns a shallow copy of this <tt>LinkedList</tt>. (The elements
     * themselves are not cloned.)
     *
     * @return a shallow copy of this <tt>LinkedList</tt> instance
     */
    public Object clone() {
        LinkedList<E> clone = null;
        try {
            clone = (LinkedList<E>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }

        // Put clone into "virgin" state
        clone.header = new Entry<E>(null, null, null);
        clone.header.next = clone.header.previous = clone.header;
        clone.size = 0;
        clone.modCount = 0;

        // Initialize clone with our elements
        for (Entry<E> e = header.next; e != header; e = e.next)
            clone.add(e.element);

        return clone;
    }

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list
     * in proper sequence
     */
    public Object[] toArray() {
        Object[] result = new Object[size];
        int i = 0;
        for (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;
        return result;
    }

    /**
     * Returns an array containing all of the elements in this list in
     * proper sequence (from first to last element); the runtime type of
     * the returned array is that of the specified array.  If the list fits
     * in the specified array, it is returned therein.  Otherwise, a new
     * array is allocated with the runtime type of the specified array and
     * the size of this list.
     *
     * <p>If the list fits in the specified array with room to spare (i.e.,
     * the array has more elements than the list), the element in the array
     * immediately following the end of the list is set to <tt>null</tt>.
     * (This is useful in determining the length of the list <i>only</i> if
     * the caller knows that the list does not contain any null elements.)
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     * <p>
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list
     * @throws ArrayStoreException  if the runtime type of the specified array
     *                              is not a supertype of the runtime type of every element in
     *                              this list
     * @throws NullPointerException if the specified array is null
     */
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[]) java.lang.reflect.Array.newInstance(
                    a.getClass().getComponentType(), size);
        int i = 0;
        Object[] result = a;
        for (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;

        if (a.length > size)
            a[size] = null;

        return a;
    }

    private static final long serialVersionUID = 876323262645176354L;

    /**
     * Save the state of this <tt>LinkedList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The size of the list (the number of elements it
     * contains) is emitted (int), followed by all of its
     * elements (each an Object) in the proper order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out size
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (Entry e = header.next; e != header; e = e.next)
            s.writeObject(e.element);
    }

    /**
     * Reconstitute this <tt>LinkedList</tt> instance from a stream (that is
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();

        // Read in size
        int size = s.readInt();

        // Initialize header
        header = new Entry<E>(null, null, null);
        header.next = header.previous = header;

        // Read in all elements in the proper order.
        for (int i = 0; i < size; i++)
            addBefore((E) s.readObject(), header);
    }
}
